The present invention relates to fasteners in general, and, more in particular, to fasteners that lock when set and that develop a predetermined clamp-up load while being set.
The venerable threaded fasteners consist of a nut and a bolt. The nut has internal threads that thread onto external threads of the bolt. Wrenching surfaces of the nut and bolt accept wrenches that tightly join the fasteners and one or more workpieces together. Broadly, another name for a bolt is a threaded pin, and another name for a nut is a collar.
Many environments in which fasteners are used require that the fasteners have extremely high integrity and strength. Fasteners must bear loads not only along their longitudinal axis but radially of the axis. More particularly, when fasteners join together two or more sheets and the sheets are loaded in their planes with different loads, one sheet tends to slide over the other. Fasteners passing through both sheets become loaded in shear during their resistance to this type of loading. Axial loads arise by the clamping of fastened sheets between a head of the pin on one side of the sheets and the collar on the other side of the sheets.
Fasteners quite often must respond well in environments where they are cyclically stressed under conditions that could give rise to fatigue failure. A fastener with adequate clamp-up load on it tends to resist fatigue failure.
An obviously desirable feature of a fastener is that it does not come apart in service. Various devices have been used to keep a collar and a pin together. One way of locking the collar and pin is to deform the threads of the collar so that they bear in radial compression against the threads of the pin. The resistance to unthreading is purely frictional. The threads are commonly deformed at the factory in preference to the field, but field deformation has also been practiced.
It is also highly desirable to know just what clamping load the fastener applies to a structure. Clamp-up load correlates to the resistance of a collar to further threading onto a pin. As clamp-up force increases, the resistance to further threading increases and the torque required to turn the collar increases. This fact has been used in fasteners to develop a predetermined clamp-up load. In one prior art fastener, a wrenching section connects to a collar by a frangible breakneck that breaks upon the application of a predetermined torque that corresponds to the desired clamp-up load.
The features of a thread lock and a collar with a frangible breakneck for clamp-up load control have been combined in one collar. Regrettably, the combination has its shortcomings. A thread lock by deforming threads of the collar is effected at the factory. As such, the collar does not freely thread onto the shear pin. This makes setting somewhat difficult. Protective and lubricating coatings applied to the threads of the collar can be worn off in a collar having this type of thread lock by the considerable frictional drag between the threads of the collar and pin. The fact that a wrenching section separates from a threaded section of the collar creates a spare piece of the wrenching section that must be removed from the environment where the fastener is set. This is a nuisance. Where corrosion control is important, a circular band of bare material on the collar is created by failure of the breakneck. This band is not protected by corrosion inhibitors applied to the fastener when it was manufactured. This type of fastener is also comparatively expensive because it requires considerable machining in its formation. The frangible breckneck section must be of very close dimensional tolerance if reasonably close tolerances in breakoff torques are to exist. This problem is compounded by machine tool wear in the tools that make the part and also because the breakneck section becomes elliptical-shaped after the thread locking feature has been incorporated. Alternative methods of forming the frangible breakneck, such as roll forming, are not available because the part is hollow. The frictional drag between the shear pin and the collar in a fastener system employing a preexisting deformed thread lock results in a broad range in clamp-up force because the drag varies between large limits and is an important component in the resistance that effects failure of the frangible breakneck.
A second approach to a locking fastener system employs a pin that has an outer annular groove placed to be outside but near to one side of the structure being fastened. A collar is deformed radially inward into the groove so that collar material is restrained axially by radial walls of the groove. A condition of axial interference exists. In one type of such fastener a female threaded member threads onto a male threaded member with one setting tool. A second setting tool radially deforms the female member into the threads of the male threaded member.